In recent years, in factories, electric power systems have been provided that use both electric power supplied via power lines from a power company, and also electric power generated by a power generator driven by a gas engine installed within the factory. Gas engines have a high energy efficiency when they conduct combustion in a fuel lean state, and are suitable for use in power generation equipment operating with a constant load. However, in gas engines, abnormal combustion such as knocking may occur during its operation. Knocking is a phenomenon in which an uncombusted air-fuel mixture is self-ignited in a combustion stroke and combustion occurs before the flame propagates or the spark timing. A pressure wave generated at the time of self-ignition imposes a burden on the engine, and as a result, the engine could be damaged. For this reason, it is necessary to properly control engine power output to suppress the occurrence of knocking with a specified intensity or higher.
Japanese Laid-Open Patent Application Publication No. 2007-247569 discloses a knocking control system for preventing occurrence of knocking in a gas engine. This control system calculates an occurrence ratio with which knocking to be evaluated occurs for set cycles, and reduces a fuel feed amount to the corresponding cylinder if the calculated occurrence ratio of the knocking exceeds a predetermined occurrence ratio. In accordance with this control system, it is possible to suppress occurrence of knocking in a gas engine.